Investigating the friction stir additively manufactured AA2024 build and the influence of material flow in enhancing the inter-surface bonding

Abstract

Friction Stir Additive Manufacturing (FSAM) is a solid-state joining process that operates below the material’s melting point, avoiding the drawbacks of fusion-based methods. In FSAM, inter-surface bonding is crucial for the build strength and is influenced by material flow at the interface. This study examines the influence of flat-faced (square) and smooth (threaded conical) pins, along with rotational speed, on the material flow, using Al-clad as a tracer. It also analyses the microstructure, micro-texture, mechanical properties of the builds, and the role of the forging force in FSAM. Defect-free builds were achieved with a forging force above 7000 N. The threaded conical pin caused upward migration of Al-clad, while the square pin led to fragmentation and distribution, offering excellent inter-surface bonding. Refined grain structures (∼5 µm) were observed in the top layers. The highest tensile strengths were 392 MPa in the weld direction (80 % build efficiency) with the threaded conical pin, and 358 MPa in the build direction (75 % build efficiency) with the square pin. Strong γ-fiber in ODFs indicates better ductility, with up to 60 % improvement in elongation. The current research shall guide the selection of appropriate process parameters and, in turn, reduce trial-and-error during manufacturing aluminum alloys through friction stir additive manufacturing.